1. Field of the Invention
This invention relates to an air-fuel ratio control system and method for an internal combustion engine and an engine control unit, for controlling an air-fuel ratio of a mixture supplied to the engine.
2. Description of the Related Art
Conventionally, there has been disclosed an air-fuel ratio control system e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2004-360628. In this air-fuel ratio control system, when an O2 sensor is active, the amount of fuel to be supplied to the engine is controlled based on an exhaust air-fuel ratio detected by the O2 sensor, whereby air-fuel ratio feedback control is carried out. On the other hand, when the O2 sensor is not active during the start of the engine, the air-fuel ratio feedback control is not carried out, but the amount of supply fuel is controlled without being based on the exhaust air-fuel ratio to control the air-fuel ratio by open control.
As described above, according to the conventional air-fuel ratio control system, the amount of supply fuel is controlled without being based on the exhaust air-fuel ratio during the start of the engine, so that e.g. when fuel difficult to burn is used, the output power from the engine is lowered, which causes degradation of a combustion state and drivability. To avoid such inconveniences, it is considered that a target value of the air-fuel ratio is set to the rich side. In this case, however, the output power from the engine becomes too large when fuel is easy to burn, whereby exhaust emissions are increased and drivability is degraded. Further, when the exhaust emissions are increased, a catalyst containing a large amount of a noble metal has to be used for reducing the exhaust emissions, which results in the increased manufacturing costs of the catalyst.